The present invention relates to an integrated circuit comprising an analogue circuit and having means to reduce the impact of substrate noise on signals in the analogue circuit, and has application particularly, but not exclusively, to mixed signal integrated circuits comprising analogue and digital circuits in which the digital circuits generate substrate noise.
The switching of logic gates in a digital integrated circuit can cause large transient currents to flow in the power supply rails within the integrated circuit. These transient currents constitute noise on the power supply rails. Digital circuits are robust in the presence of such noise but, in a mixed signal integrated circuit, if analogue circuits use the same power rails this noise can corrupt the analogue signals.
The problem of mixing analogue and digital circuits on the same integrated circuit will be described with reference to FIG. 1. FIG. 1 is a schematic diagram showing an integrated circuit chip 100 comprising an analogue circuit 30 and a digital circuit 50. The digital circuit 50 comprises CMOS logic gates. The switching of the CMOS logic gates causes large transient currents to flow into a power source 300 via bond wire inductances 101, 103. The flow of the transient currents in the bond wire inductances causes a disturbance, termed substrate noise, on the on-chip digital supply rails 112, 114 which operate at voltages Vddd and Vssd respectively. If the digital supply rails were to be used by the analogue circuit this disturbance would corrupt the analogue signals in the analogue circuit. Disturbance to Vddd can be prevented from corrupting the analogue signals by supplying the analogue circuit from a separate supply rail 110 supplying voltage Vdda, as shown in FIG. 1. However if the Vssd supply rail 114 is shared by both the analogue circuit and the digital circuit, disturbances to Vssd corrupt the analogue signals in the analogue circuit.
The analogue circuit may be supplied with two separate rails (not illustrated in FIG. 1) at voltages Vdda and Vssa respectively, but if the Vssa rail is connected to the substrate of the integrated circuit chip then noise in the substrate modulates the effective supply (Vdda-Vssa) changing the operating points of the analogue circuit, and also parasitic capacitances can couple the noise in the substrate into the analogue signal paths.
If the separate rail at Vssa is not connected to the substrate, then, assuming an N-well CMOS process, the analogue signals can be corrupted through the backgate effect in the NMOS transistors in the analogue circuit and via parasitic capacitances which couple the NMOS transistors to the substrate.
Balanced analogue circuits are often used to reduce the impact of the substrate noise but under large signal conditions the circuits become unbalanced and the analogue signals are corrupted. The problem is so serious that many systems are designed using separate chips for analogue and digital circuits so that they no longer share the same substrate but this makes a less cost effective solution.
Substrate noise can also be generated by analogue circuits operating at a high level, such as a power amplifier, which can corrupt signals in analogue circuits operating at low levels.
An object of the invention is to provide an integrated circuit having improved noise performance.
According to the invention there is provided an integrated circuit, comprising an analogue circuit coupled to first and second supply rails and coupling means for coupling noise on the first supply rail to the second supply rail.
By coupling the noise on the first supply rail onto the second supply rail, the noise is reproduced on both the first and second supply rails and the relative voltage differences between the first and second supply rails and between the internal nodes of the analogue circuit is substantially independent of the noise. In this way the impact of the noise on signals in the analogue circuit is reduced or eliminated.
The integrated circuit may also comprise a digital circuit coupled to the first supply rail. The digital circuit may be the source of the noise. The first supply rail may coupled to ground.
The integrated circuit may comprise only analogue circuitry, without any digital circuits, with the noise being generated by analogue circuitry, for example by current pulses flowing in the bond wire inductances 101, 102.
The coupling means for coupling noise on the first supply rail onto the second supply rail may comprise a power supply regulator supplying the second power rail arranged so that the noise on the first supply rail modulates the second supply rail.
The coupling means may further comprise a first capacitor means having first and second ports wherein the first port is coupled to the first supply rail and the second port is coupled to a control node of the power supply regulator such that the noise on the first supply rail is coupled to the control node and modulates a voltage supplied to the second supply rail by the power supply regulator.
The integrated circuit may also comprise a second capacitor means having first and second ports wherein the first port is coupled to the first supply rail and the second port is coupled to the second supply rail. By means of this second capacitor means, noise on the first supply rail is coupled to the second supply rail and, in conjunction with the first capacitor means, voltage fluctuations within a regulation device within the power supply regulator caused by noise can be reduced, thereby reducing the required bandwidth of the regulation device.